Generally, a sewing machine must include an upper horizontal arm, a lower machine bed and a needle plate. An arm shaft in the upper horizontal arm actuates a thread take-up mechanism and a crank unit synchronously, so as to drive a needle bar mechanism at a sewing head to move upward and downward. A fabric feeding mechanism is provided on the lower machine bed, and a loop taker is provided below the needle plate. In a sewing operation, a needle carried by the needle bar mechanism is driven to move upward and downward reciprocatingly to pierce through the fabric, so that a thread extended through an eye of the needle is also brought to downward extend through the fabric; meanwhile, the thread take-up mechanism and the loop taker are coordinately moved in correct time sequence to take up and release the thread.
The thread take-up mechanism is normally arranged above the needle bar mechanism to ensure proper tension of the thread and minimize a resistance to the thread when the same passes through the thread take-up mechanism. In the market, except some multi-needle bar embroiders, there are few sewing machines that allow direct switching of the needle bars thereof. Since the multi-needle bar embroiders generally have the drawbacks of having a bulky volume and requiring an extended travel distance for a fabric feeding surface on a driving unit, it is necessary to develop a sewing machine that allows direct switching of multiple needle bars from one to another according to actual need in the sewing operation while the sewing reference point and the needle plate for those needle bars are unchanged.
However, once the sewing machine adopts a cylindrical needle bar case on the sewing head to enable needle bar switching operation, the thread take-up mechanism must also be able to handle multiple threads. In this case, if the thread routing were not well controlled, the length of the threads in the thread course can not maintain same after the needle bar switching operation, resulting in change of the tension of the threads to adversely affect the subsequent sewing operation.